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The Dreaded Diatom Bloom

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I wanted to write about something that is often discussed, but never fully explained. I am talking about diatoms.

Diatoms are a member of the Kingdom Protista. Newer hobbyists are usually led to believe that the word diatom is used to describe one specific species of algae. However, the class Bacillariophyceaea (diatoms) includes more than twenty thousand species of algae. That is, if you tell a new hobbyist that you have had a bloom of the same algae, you might just be lying. Diatoms have a few subtle differences from the stereotypical alga. The first difference in diatoms is that they have an outer shell composed of silica. As humans, we benefit from these shells every day. The silica shells of diatoms are made into things such as toothpaste, paint, polish, soap, and even ice cream. Secondly, diatoms have a unique and interesting way of reproducing which I will outline later in this article.

To begin, let’s discuss why diatoms are bad, where they come from, why they appear, and how to get rid of them.

Diatoms Are Bad?

Almost every hobbyist will experience an outbreak of diatoms during the months that their aquarium is cycling. For new hobbyists, seeing brownish-red or rust colored algae cover their aquarium leads them to the assumption that it is bad. After seeing this growth, they will seek help, usually to get the advice that it is, “Just a diatom bloom” and that it will, “go away in a few weeks.” There are three ways in which diatoms can be bad for your aquarium. The first way that they can be bad, is that they are unsightly. The second way that they are bad is that when they die, they will cause a drop in dissolved oxygen levels. The third way that they can be a nuisance is rarer than the first two.
Certain species of diatoms are encrusting. This means that the algae will form a thicker layer (or crust if you will) around your rocks and coral bodies. This can then suffocate and kill certain corals. Generally, the third nuisance will not occur. With that said, when the population is stable, they are very good for your aquarium. Seeing as 98% of phytoplankton is composed of diatoms, this can be a very good and natural source of food for your filter feeding “wet pets.”
There are more ways in which diatoms can be helpful. The first is that diatoms on your sand are a great food source for a large portion of mollusks and crustaceans common sold in this hobby. Secondly, diatoms consume nitrates and phosphates. When the diatoms are filtered out via protein skimming or filter feeders, the nitrates and phosphates are filtered out as well. So next time you have diatoms, don’t “get rid of them,” but rather get their populations under control.

Where Are They Coming From?

This is a simple question with a simple answer. Diatoms come from live sand or live rock that you buy. Even live rock from stable tanks that have been running for years have diatoms. These diatoms are in stable populations as they should be though, and when given the nutrient resources available in newer tanks, the population flourishes and gives the aquarium a nice coating of algae.

Why Are They Appearing?

Most hobbyists assume that diatoms appear because they are doing something wrong. This is usually not the case as even experienced hobbyists can get a bloom of diatoms when starting a new tank. As we already discussed, diatoms have an outer shell composed of silica. To form this shell, diatoms need to feed on silicates. There are two main sources that silicates come from. The first source explains why diatoms occur during the first weeks of aquarium keeping. The first source is sand. When you buy sand, there is generally a high concentration of silicates which can cause a bloom. After all of the silicates are consumed from the sand, the population of diatoms should stabilize, but this is not always the case. The second source of silicates is your source water. If the water that you use to top off and use for water changes has silicates in it, then the population of diatoms will have an unlimited and bountiful food source and may not stabilize.
Even when using reverse osmosis and deionization, the filtered water may contain silicates. It doesn’t take very long for the silicates in your tap water to clog the membrane and allow more silicates through. So if you get a diatom bloom in a mature tank or if your diatom bloom does not go away, then you will need to check (or change) your source water and source water filters. I should also mention that certain types of sand that are not made for aquariums can have very high levels of silicates and silicic acid which will cause a never ending diatom bloom. These sands should be avoided unless you are sure that they are aquarium safe.

How Do I Get Rid of Them?

This question is asked much too often and the answer to it is usually very simple. To get rid of diatoms, you just wait. I do not like this answer, although it isn’t wrong. To get rid of diatoms, you need to get rid of the silicates in your water. Since diatoms are consuming the silicates, you can simply remove the diatoms from the water and the problem should eventually go away. With frequent water changes, the remaining silicates in the water will be removed. After all, “dilution is the best solution to pollution.” Some people take advantage of the diatom bloom and begin adding their clean up crew at this time.
Since diatoms are such a good source of food, your pets get fed as your problem gets solved. Be sure to not add too many members to your clean up crew though, you do not want them to starve after all the diatoms are gone. Some good diatom cleaners that I recommend are nassarius, cerith, and astrea snails. However, there are several organisms that consume diatoms, so choose the ones that are best for you. If you are running an aggressive tank and such invertebrates won’t work for you, then there are other ways of ridding your tank of diatoms. You can remove the diatoms by hand and you can also buy a diatom filter pad. There are several products out there that can be used to filter out silicates. You want to be sure that you get these diatoms out of your water, because if they die, they can cause oxygen levels to decrease and become food for other nuisance algae and bacteria.

How Do They Reproduce?

Diatoms can reproduce very quickly and as I said before, there means of reproduction is something very unique. Diatoms reproduce both sexually and asexually. When culturing diatoms in a Petri dish, I realized that diatoms look somewhat like the Petri dish itself. Diatoms are circular disk-like organisms with two halves (like the lid and the base of a Petri dish). These two pieces will split apart asexually and form another half. Each new half will function as a “lid” and grow a base. Since the base is smaller than the lid in diameter, the new diatom will be smaller than its parent. As a matter of fact, diatom comes from the word diatomos meaning cut in half. After several generations of reproduction, the diatoms will eventually become too small to reproduce asexually. When this happens, the diatom will produce either sperm or eggs and release this into the water column. When the sperm meets the egg, a new generation of diatoms will form, this time much larger.
This asexual reproduction occurs very quickly if there are enough nutrients for it to feed on. The entire aquarium could be completely covered in these algae in a matter of a few days if the nutrients are available. If there are not enough resources for these algae to survive on, they can form endospores. Basically, an endospore is an inactive organism. Diatoms form endospores when there are not enough available nutrients or when conditions are not favorable for them. When the conditions do become favorable, they can spring back to life and reproduce astronomically. This is just something to be aware of.

I hope this article has been somewhat educational and has opened your eyes to how overlooked diatoms are in this hobby. They are an important, if not necessary part of the hobby and are usually dubbed the role of one of the bad alga.